During the day, as the slopes are heated there is upslope flow moving towards higher elevations--this upslope flow and the destabilization of the atmosphere as the mountains heat up produces convection and enhanced upward flow. Sometimes this convection is accompanied by a cumulus cloud as well as smoke (called pyrocumulus). The rising air cools due to the expansion of the upward-moving air and eventually the initially warm air is no longer buoyant (becomes the same temperature as the environmental air at that level) and no longer rises. You see this all the time from smoke from smokestacks hitting a level through which it can pass and then spreads mainly horizontally--check out this picture:

During the evening, the surface of the mountains cools as the infrared radiation loss to space exceeds the incoming energy from the sun. The daytime convective/smoke plume dies (since it depends on the surface being sufficiently warmer than the air above), and air starts moving down the mountain....known as downslope flow. The smoke is entrained in this flow and heads down the slope. That is what you are seeing.